BARTINGTON INSTRUMENTS
Page 18 of 32 OM2020/5
3. Rotate to the next pair of levelling screws and repeat the adjustment.
4. Rotate and continue adjustment until the desired accuracy is achieved.
6.3. Performing Observations of D&I with A Non-Resettable Horizontal Circle
Theodolite
The re-settable circle previously provided on the steel equivalent theodolite is not required for
accurate measurement of D and I and has been eliminated. The likelihood of incorrect circle
settings is reduced and the best observation method is adopted.
Note:
A digital calculator will be required which can convert degrees, minutes and seconds
to decimal values and vice-versa. Alternatively, computer programs are available to assist
with data reduction.
An accurate digital clock with a large display positioned away from the theodolite is also
required.
The observation conventions are described pictorially in Figures 4, 5 and 6.
Two optical and four magnetic measurements are performed for each observation, and the
results are averaged to eliminate system optical and magnetic errors. Worked examples are
given at the end of this section. The examples are for explanation only and do not show the
system errors.
6.4. Taking Readings
Note:
After levelling the theodolite, the vertical and horizontal scales will be in a random
position.
1. Direct the theodolite at the reference target or a null field position. After aligning in the
direction of interest the circle graduation lines are unlikely to be exactly in the middle of the
double line index, and will more likely resemble those displayed in Figure 7, below.
2.
Micrometer
Scale
